In recent years, the technique for TFTs for liquid crystal display devices has progressed, and servicing for information infrastructure has been made satisfactory. Thus, at the present time, a flat panel detector (hereinafter referred to as “an FPD” for short when applicable) is proposed which is obtained by combining a sensor array constituted by photoelectric conversion elements using non-monocrystalline silicon, e.g., amorphous silicon (hereinafter referred to as “a-Si” for short) and switching TFTs with a phosphor for converting radiation into light such as visible light. Also, even in the medical image field, the FPD can have a large area, and digitization of the FPD is attained.
This FPD is adapted to read out a radiation image in an instant to display the radiation image thus read out on a display device simultaneously, and an image can be directly fetched in the form of digital information from the FPD. Thus, the FPD has the feature that handling management is convenient in safekeeping of data, or process and transfer of data. In addition, it was verified that though the characteristics such as sensitivity depend on photographing conditions, the characteristics are equal to or superior to those in a conventional screen film (hereinafter referred to as “S/F” for short) photographing method or a computed radiography (hereinafter referred to as “CR” for short) photographing method.
An equivalent circuit diagram of 1 bit in this FPD is shown in FIG. 10. In addition, a schematic equivalent circuit diagram of (3×3) bits in the FPD is shown in FIG. 11. In these figures, reference numeral 101 and reference symbols S11 to S33 designate photoelectric conversion element portions, respectively; reference numeral 102 and reference symbols T11 to T33 designate transferring TFT portions, respectively; reference numeral 104 and reference symbols Vg1 to Vg3 designate transferring TFT driving wirings, respectively; reference numeral 106 and reference symbols Sig1 to Sig3, signal lines; reference numeral 107 and reference symbols Vs1 to Vs3, photoelectric conversion element biasing wirings; reference symbol A, a signal processing circuit; reference symbol B, a bias power source; reference symbol D, a TFT driving circuit; and C, an A/D converter.
Radiation such as X-rays is made incident from an upper side in the paper of FIG. 11 to be converted into light such as visible light by a phosphor (not shown). The resultant light is then converted into electric charges by the photoelectric conversion elements S11 to S33 to be accumulated in the photoelectric conversion elements S11 to S33. Thereafter, the transferring TFT portions T11 to T33 are operated by the TFT driving circuit D through the TFT driving wirings so that these accumulated electric charges are transferred to the signal lines Sig1 to Sig3 to be processed in the signal processing circuit A. Moreover, the resultant signal is then subjected to A/D conversion in the A/D conversion unit C to be outputted.
Basically, the element structure as described above is generally adopted. In particular, as for the above-mentioned photoelectric conversion element, a PIN type photo detector (hereinafter referred to as “a PIN type PD” for short when applicable), or an MIS type photo detector (hereinafter referred to as “an MIS type PD” for short) adopted by the present inventors or the like is generally used. In addition to such elements, various kinds of elements have been proposed.
As described above, commercialization of the FPD has been attained. On the other hand, various proposals for the FPD have been made for the purpose of aiming at further enhancing the sensitivity. For example, in a report made in a literature of L. E Antonuk et al.: “SPIE Medical Imaging VI”, February, pp. 23 to 27, 1992, there is disclosed a structure in which a sensor element is laminated on a TFT element. In this proposal, adoption of the above-mentioned structure allows an open area ratio of the sensor element to be increased to make enhancement of sensitivity possible. In addition, it is described that since the TFT element is disposed right under the sensor element, an unnecessary parasitic capacity is formed, and hence a grounded plane is disposed. However, specific details are unclear, and so are effects thereof.
In addition, in a proposal made in a literature of the specification in U.S. Pat. No. 5,498,880, likewise, there is shown a structure in which in order to increase an open area ratio, a sensor element is laminated on a TFT element. In this proposal, there is adopted the structure in which an electrode connected to a source/drain electrode of the TFT element covers the TFT element, and also becomes a separate electrode of the sensor element.
On the other hand, in a proposal as well in a literature of Japanese Patent Application Laid-Open No. 2000-156522, there is shown a structure in which for the purpose of aiming at increasing an open area ratio, a sensor element is laminated above a TFT element. In this proposal, there is adopted the structure in which the sensor element is laminated over the TFT element through an interlayer film.
However, in the above-mentioned related art examples, there is adopted the structure such that the channel portion of the TFT suffers the fluctuation in electric potential in various forms. In addition, even in the description that a shielding layer is disposed, its structure is not concretely clear.
That is to say, in the FPD having the conventional lamination structure, the separate electrode of the sensor element acts as a back gate electrode of the TFT element. Thus, a problem such as generation of a leakage current of the TFT element is caused due to the fluctuation in electric potential of the separate electrode to cause degradation in image quality.
In a case where for example, an area having a large sensor output signal, and an area having a small sensor output signal are disposed adjacent to each other, such crosstalk as to blur a boundary between these areas appears. In addition, there is caused a problem that a sensor saturation output is decreased to reduce a dynamic range.